This application is directed to an apparatus and process which can be used in producing at least some of the products described in copending patent applications, xe2x80x9cThe Deposition of Resistor Materials Directly on Insulating Substratesxe2x80x9d, Ser. No. 09/069,679, now U.S. Pat. No. 6,210,592; xe2x80x9cResistors for Electronic Packagingxe2x80x9d, Ser. No. 09/069,427, now U.S. Pat. No. 6,208,234; and xe2x80x9cPrecursor Solution Compositions for Electronic Devices Using CCVDxe2x80x9d, Ser. No. 09/069,640, now U.S. Pat. No. 6,193,911; and xe2x80x9cControlled Atmosphere Flame For CCVD Processxe2x80x9d, Ser. No. 09/067,975, pending filed on or about the same day as this application, the contents of which applications are incorporated by reference herein.
1. Field of the Invention
This invention relates to chemical vapor deposition (CVD) wherein coatings are applied to substrates by reacting a coating precursor in a reaction zone to produce a reaction product of the coating precursor which immediately contacts a substrate forming a coating thereon. The invention is particularly directed to improvements in CVD apparatus and processes which permit the production of high quality thin film coatings on temperature sensitive substrates without the need for creating such coatings in a vacuum or similar chamber. In preferred embodiments the invention enables the production of thin film coatings on temperature sensitive substrates at atmospheric pressure, thereby enabling the production of high quality thin film coatings on large substrates which could not be coated by prior techniques requiring vacuum processing.
2. Description of Related Art
Chemical vapor deposition (CVD) is a well known technique for depositing coatings by providing a gaseous reactant material which reacts adjacent to, or on, a substrate surface to produce a solid deposit or coating on that surface. A recent development of the CVD process, referred to as Combustion Chemical Vapor Deposition, or CCVD, is described in U.S. Pat. No. 5,652,021, and is incorporated by reference herein. The reactants in that process are fed dissolved or suspended in a liquid, which can be a fuel, and which is sprayed into a reaction zone from a nozzle using an oxidizing gas as the propellant. The sprayed mixture is either ignited producing a flame, or is introduced into a flame, while a substrate is maintained near the flame""s end. The reactants, which vaporize either prior to or in the flame, produce a deposited film on the substrate. The patent describes a number of prior CVD processes, including some which feed gaseous or vaporized reactants, some which use a sprayed or atomized solution, and some which feed reactive solid powders. The patent also describes a number of alternative coating techniques including spray pyrolysis wherein solutions are sprayed onto a heated substrate where they pyrolyze to form a coating, and techniques wherein a solid coating material is either melted or vaporized in a flame, plasma or other heating device and splattered or condensed on a substrate to form a coating.
One embodiment described in the patent involves providing a coating which requires a reducing atmosphere on a substrate deployed in the reducing region between the inner and outer flames produced by a Smithell separator. The techniques described in this patent have been generally employed to successfully provide coatings of oxides and a few relatively oxidation resistant metals. However, the production of quality coatings of many metals and other relatively oxidation susceptible materials has been inconsistent prior to the development of the present invention.
While a number of materials can be deposited from a reducing flame, there are numerous materials which can only be deposited in the absence, or near absence, of oxygen. Most nitrides, carbides and borides require an oxygen free environment, not only free of free oxygen, but also free of combined oxygen in such as water and carbon oxides. Those elements which are more susceptible to oxidation, such as aluminum, silicon and titanium, also require an oxygen free atmosphere. Embodiments of the invention disclosed herein enable the deposition of such oxygen sensitive materials.
Moreover, there is interest in developing techniques for forming thin coatings of low dielectric constant materials as interlayers on temperature sensitive substrates, such as electronic chips, condensers and microcircuit laminates. Polymers, particularly polyfluorocarbons, such as polytetrafluoroethylene, and polyimides, are of particular interest because of their low dielectric constant and high thermal stability. Coatings of these and other organic materials are also potentially useful for corrosion, optical, thermal, cosmetic, wear and release property applications. The inventive process enables coatings of these polymers to be applied from their monomeric or low molecular weight precursors onto substrates which are temperature and/or oxidation sensitive.
A further improvement of the CCVD process is described in U.S. patent application Ser. No. 08/691,853, filed Aug. 2, 1996, and which is hereby incorporated by reference. This application describes a CCVD process wherein the coating precursor reactant is provided in admixture or solution in a liquid feed stream which is pressurized to near its critical pressure and heated to near its supercritical temperature before being directed through a nozzle or other restriction. The near-critical conditions of the liquid result in the feed stream being very finely atomized or vaporized as it is leaves the nozzle to enter a zone where the coating precursor reacts and either deposits a coating on a substrate or is recovered as a finely divided powder.
This invention provides an apparatus and method for chemical vapor deposition wherein the atmosphere in a controlled atmosphere zone is established by carefully controlling and shielding the materials fed to form the coating and by causing the gases removed from the controlled atmosphere zone to pass through a barrier zone wherein they flow away from said controlled atmosphere zone at an average velocity greater than 50 feet per minute, and preferably greater than 100 feet per minute. The controlled atmosphere zone is inclusive of the reaction zone, wherein the coating precursor is reacted, and the deposition zone, wherein the reaction product of the coating precursor deposits a coating on a substrate. The rapid gas flow through the barrier zone essentially precludes the migration of gases from the ambient atmosphere to the deposition zone where they could react with the coating, the materials from which the coating is derived, or the substrate.
Careful control of the materials used to form the coating can be provided by feeding the coating precursors in a fixed proportion in a liquid media. The liquid media is atomized as it is fed to a reaction zone wherein the liquid media is vaporized and the coating precursors react to form reacted coating precursors. Alternatively, the coating precursor(s) can be fed as a gas, either as the pure coating precursor or as a mixture in a carrier gas. The reacted coating precursors can be composed of partially, fully and/or fractionally reacted components, which flow to the substrate. The reacted coating precursors contact and deposit the coating on the surface of the substrate in the deposition zone. A curtain of flowing inert gases may be provided around the reaction zone to shield the reactive coating materials plasma in that zone from contamination with the materials used in the surrounding apparatus or with the components of the ambient atmosphere.
The vaporization of the liquid media and reaction of the coating precursors in the reaction zone requires an input of energy. Depending on the reactivity of the coating material and the substrate, the required energy can be provided from various sources, such as combustion, electrical resistance heating, induction heating, microwave heating, RF heating, hot surface heating, laser heating and/or mixing with a remotely heated gas.
For coating applications which do not require an oxygen free environment, an embodiment of the present inventive apparatus which incorporates the recently developed Combustion Chemical Vapor Deposition (CCVD) process, as described in the incorporated U.S. Pat. No. 5,652,021, is particularly advantageous. We refer to this process as Controlled Atmosphere Combustion Chemical Vapor Deposition (CACCVD). This technique provides a relatively high rate of energy input, enabling high rates of coating deposition. In some preferred cases, the fluid media and/or a secondary gas used to atomize the fluid media can be a combustible fuel which also serves as an energy source. Particularly important is the capability of CACCVD to form high quality adherent thin film deposits at or about atmospheric pressure, thereby avoiding the need for elaborate vacuum or similar isolation housings. For these reasons, in many cases, CACCVD thin film coatings can be applied in situ, or xe2x80x9cin the fieldxe2x80x9d, where the substrate is located.
Combustion chemical vapor deposition (CCVD) is not suitable for those coating applications wherein the coating, and/or the substrate, require an oxygen free environment. For such applications, embodiments of the present invention employing non-combustion energy sources such as hot gases, heated tubes, radiant energy, microwave and energized photons, as with infrared or laser sources, are suitable. In these applications it is important that all of the liquids and gases provided to the reaction and deposition zones be oxygen free. The coating precursors can be fed in solution or suspension in liquids. Liquid ammonia and propane are suitable for the deposit of nitrides or carbides, respectively. The use of these non-combustion energy sources in a controlled atmosphere chemical vapor deposition system which forms deposits at or above atmospheric pressure is a particularly advantageous and unique embodiment of this invention. The use of the non-combustion energy sources in a CVD system which provides enhanced atomization by the rapid release through a nozzle, or similar restriction, of the liquid coating precursor from near critical temperature and pressure conditions is a further uniquely advantageous embodiment.
The embodiments of the invention which use non-combustion energy sources are also particularly suitable for applying organic coatings. These coatings generally require less energy input than is usually involved with inorganic coatings. Moreover, organic materials generally have relatively low to moderate decomposition temperatures requiring careful control over the energy input and achieved temperatures. Accordingly, embodiments of the invention which incorporate such energy sources as mixing with remotely heated liquids or gases, hot-surface heating, electrical resistance heating, induction heating, and heating methods employing RF, infrared or microwave energy, are well suited for depositing organic coatings.
Since the inventive process and apparatus provide a controlled atmosphere zone which is capable of movement relative to the substrate, it enables the production of coatings on substrates which may be larger than the controlled atmosphere zone and, therefore, larger than could otherwise be processed by conventional vacuum chamber deposition techniques.
A further advantage of the present system is its ability to coat substrates without needing additional energy supplied to the substrate. Accordingly, this system allows substrates to be coated which previously could not withstand the temperatures to which substrates were subjected by most previous systems. For instance, nickel coatings can be provided on polyimide sheet substrates without causing deformation of the substrate. Previously, atmospheric pressure deposition techniques were unable to provide chemical vapor deposition of metallic nickel because of its strong affinity to oxygen, while vacuum processing of polymeric sheet substrates, such as polyimide sheets, was problematical due to its causing of outgassing of water and organic materials, and such substrates tendency toward dimensional instability when subjected to heat and vacuum.